In a conventional illuminating lamp, electrical discharge within the transparent bulb causes heating of the electrodes or tungsten filaments, whereby the metals of which the electrodes or filaments are formed are evaporated, and the evaporated metal is deposited on the entire interior wall surface of the bulb and causes a darkening of the bulb, i.e. a reduction in the amount of light transmitted through the bulb. If the electrodes or filaments contain volatile matter which is emitted during the time of operation of the lamp within the bulb, this results in contamination or turbidity of the lamp.
In either case, the quantity of light transmitted from the light source through the bulb is reduced by about 10% at the end of 1000 hours of lighting operation. In consequence, this type of lamp is not suitable for use as a light source for various optical measuring apparatuses such as light fastness or weathering test apparatuses that require a stable source of a constant quantity of light.